Abstract

α-(Yb1-xErx)2Si2O7 thin films on Si substrates were synthesized by magnetron co-sputtering. The optical emission from Er3+ ions has been extensively investigated, evidencing the very efficient role of Yb-Er coupling. The energy-transfer coefficient was evaluated for an extended range of Er content (between 0.2 and 16.5 at.%) reaching a maximum value of 2 × 10−16 cm−3s−1. The highest photoluminescence emission at 1535 nm is obtained as a result of the best compromise between the number of Yb donors (16.4 at.%) and Er acceptors (1.6 at.%), for which a high population of the first excited state is reached. These results are very promising for the realization of 1.54 μm optical amplifiers on a Si platform.

1. Introduction

During the last two decades Er-doped Si-based systems have attracted a considerable attention as materials for the development of efficient sources to be used in silicon microphotonics [1]. The Er emission wavelength of about 1.54 μm is in fact very important, since it corresponds to a minimum in the loss spectrum of silica optical fibers. The Er content that can be inserted in solid hosts is however limited to about 1020 at/cm3, due to the occurrence of segregation or clustering phenomena, that limit the number of emitting centers [2]. A promising alternative approach is represented by Er-based compounds, such as oxides or silicates, in which Er can be inserted in higher concentration [3–6], about 1022 at/cm3, by avoiding the Er-Er clustering. It has been demonstrated that all Er ions are optically active in Er2Si2O7 [3]. However extremely high Er concentration introduces also strong non-radiative recombination paths, such as concentration quenching and cooperative up-conversion, that limit the 1.54 μm luminescence. Yttrium-Erbium silicates have been suggested in order to introduce gradually the Er content by reaching the best compromise between a high number of optically active centers and the occurrence of the deleterious non-radiative paths [7–9]. In these systems a very low cooperative up-conversion coefficient, about 10−18 cm3/s, has been found for 1021 Er/cm3 [8, 9], thus demonstrating the possibility to reach high optical gain potentialities.

Another common technique, successfully used in diode pumped glass lasers, and especially in fiber lasers [10–13], for increasing the amount of excited Er3+ ions in the 4I13/2 level is the Yb3+ sensitization. Thanks to the energy match between the 4I11/2 level of Er3+ and the 2F5/2 level of Yb3+, the energy transfer between an excited Yb3+ ion and an Er3+ ion in the ground state can occur resonantly. This process is very efficient since the absorption cross section of Yb3+ (about 10−20 cm2) is one order of magnitude higher than that one of Er3+ (about 10−21 cm2).

In this work we have studied the Yb-Er couple in (Yb-Er) disilicate, α−(Yb1-xErx)2Si2O7, thin films grown on Si. The choice takes advantage of the Er-based compounds (for inserting controlled Er concentrations), of the efficient Er3+ sensitization from Yb3+ ions and of the compatibility with the Si platform. The disilicate host has also the advantage of a high phonon energy (≈1100 cm−1). This leads to a high 4I11/2→4I13/2 transition probability, thus reducing the back-energy transfer from Er3+ to Yb3+ and guaranteeing a high population of the 4I13/2 level. The Er concentration, NEr, was varied between 0.2 at.% and 16.5 at.%, by keeping constant the total RE concentration (NEr + NYb = 18 at.%), in order to study the Er3+ excitation and de-excitation mechanisms and the Yb-Er coupling in the α−(Yb1-xErx)2Si2O7. Moreover two sets of reference samples, α−(Y1-xErx)2Si2O7 and α−(Yb1-xYx)2Si2O7, have permitted to evaluate the optical properties of the single RE without the influence of the second one. These reference samples are very important since Y3+ has the same chemical properties of a RE but it is optically inactive. In our best sample, a very high energy-transfer coefficient has been reached, about 2 × 10−16 cm−3s−1. The best compromise between the number of excited Yb3+ donors and the available Er3+ acceptors is obtained for 1.6 at.% of Er (corresponding to 1.4 × 1021 Er/cm3). At this concentration a very high excited population at the 4I13/2 level has been estimated suggesting that material is very promising as an active medium for planar optical amplifiers at 1535 nm.

2. Experimental section

We have deposited (Yb1-xErx)2Si2O7 thin films, about 150 nm thick, in a ultra high vacuum magnetron sputtering system on (100) c-Si substrates by the co-sputtering from Yb2O3, Er2O3 and SiO2 targets. (Y1-xErx)2Si2O7 and (Yb1-xYx)2Si2O7 thin films have been also synthesized, as reference samples, by substituting respectively the Yb2O3 or the Er2O3 with the Y2O3 target. Further details on the apparatus and on the deposition procedure can be found elsewhere [9]. By properly changing the powers supplied to the three targets we have synthesized several films having different ratio between the two involved REs but keeping always fixed their sum, 18 at.%, and the disilicate composition RE:Si:O = 2:2:7. All the films have been treated at 1200 °C for 30 s in O2 ambient in a rapid thermal annealing system, in order to induce the α-phase crystallization.

Thin films composition has been evaluated by Rutherford Backscattering Spectrometry (RBS) and by Particle Induced X-Ray Emission (PIXE). RBS measurements were performed by using a 2 MeV He+ beam, with the detector placed at an angle of 165° with respect to the incident beam. PIXE analyses were realized by using a 2.4 MeV He+ beam; a higher energy with respect to RBS measurements was chosen in order to increase the ionization cross section, and hence the probability of X-ray emission. A Si-PIN covered by a thin Be window was used as a detector; an Al filter was placed in front of the detector, in order to cut the low-energy X-rays originating from the Si substrate.

Room temperature photoluminescence (PL) measurements have been performed by a Titanium-sapphire laser pumped through a laser diode at 532 nm and mechanically chopped at a frequency of 11 Hz. The wavelength of the Ti-sapphire laser has been varied with continuity between 750 and 1000 nm. The PL signal was analyzed by a single grating monochromator and detected by a germanium photodetector. Time resolved PL measurements were performed by first detecting the modulated luminescence signal with an Hamamatsu infrared-extended photomultiplier tube and then by analyzing the signal with a photon counting multichannel scaler. The overall time resolution of the system is of 1 μs.

3. Results and discussion

3.1 Structural properties

RBS analyses have been performed for all the samples and the spectra were fitted by using the SIMNRA 6.0 software [14], in order to estimate their chemical composition. Figure 1(a)
reports some examples of RBS spectra. At a first glance it is evident that all the elemental signals are constant, thus indicating a uniform chemical composition along all the film thickness.

All the compounds obtained by changing the powers applied to the Y2O3 and Er2O3 targets, and by keeping constant at 300 W the one to the SiO2 target, have the disilicate stoichiometry (Y + Er):Si:O = 2:2:7 (see the example in Fig. 1(a), green line and open triangles). A similar composition, i.e. (Yb + Y):Si:O = 2:2:7, has been confirmed for all the samples obtained by varying the powers supplied to the Yb2O3 and Y2O3 targets (not shown). In both kinds of disilicates the atomic concentration of the single RE is also evaluated, and it varies in the range between 0.2 at.% and 18.0 at.%.

In contrast in (Yb-Er) compounds it is not possible to distinguish the Yb and Er contributions to the RBS signal. This is evident by looking at the spectra in Fig. 1(a) regarding Yb2Si2O7 (blue continuous line) and (Yb-Er) compound (red line and open circles). In fact the energy difference between their surface edges is lower than the energy resolution of the detector as a consequence of their very similar masses. In this case the experimental spectra were fitted by assuming that the RE RBS signal is related only to a single RE (while in effect it is the sum of both REs). In fact Yb and Er Rutherford backscattering cross sections, σR, differ only by 5%. Since the detected RBS signal is directly proportional to σR, the highest error in the total RE estimation is 5% in the worst case. By fitting the RBS spectra we can conclude that all the as-deposited (Yb-Er) compounds have a disilicate composition (Yb + Er):Si:O = 2:2:7.

In order to evaluate independently the concentrations of each RE, we have performed PIXE analyses. Figure 1(b) reports the PIXE spectra of two reference samples Yb2Si2O7 and Er2Si2O7 and of the (Yb1-xErx)2Si2O7 compounds obtained for different powers applied to the two RE oxide targets. In the case of Yb2Si2O7 film (blue continuous line) only the Yb-related peaks are visible (Lα1,2, Lβ and Lγ as labeled in the figure). Analogously for Er2Si2O7 (black line and open squares) peaks related to Er transitions appear. Finally in the (Yb1-xErx)2Si2O7 films peaks related to both Yb and Er are present and their intensities vary depending on the content of each species. In PIXE spectra the peaks associated to Yb and Er are well separated, and then it is possible to analyze them individually.

The quantitative estimation of the single RE concentrations was done by combining RBS and PIXE measurements and by using the Yb2Si2O7 and Er2Si2O7 films as calibration standards. In fact through RBS analyses it is possible to determine the atomic areal density of the sum of Yb and Er, while PIXE allows to identify their mutual weight. In order to have the best sensitivity we have chosen the Lα1,2 peak due to its higher intensity with respect to the other ones. By using the two calibration standards we can obtain a parameter that directly correlates the RE areal density (expressed in cm−2) to the area of the Lα1,2 peak individually for each RE, equal to 3.26 × 10−3 and 3.44 × 10−3 cm−2keV−1 for Yb and Er respectively.

Measuring the areas of the Er Lα1,2 and Yb Lα1,2 peaks, reported in Table 1
, in all the as-deposited (Yb1-xErx)2Si2O7 samples we have directly extracted the atomic areal density of Yb and Er, NYb and NEr. This procedure is justified by the fact that both the ionization cross section and the fluorescence yield can be considered unchanged for all the samples because they do not depend on the film composition. This is a reasonable assumption since the atom ionization by ion impact is an atomic process, which does not involve the material structure in which the atom is embedded. In addition also the x-ray absorption of the materials is unchanged. The estimated atomic concentrations of Yb and Er have been reported in atomic percentage in Table 1, as obtained by considering the total atomic density estimated by RBS measurements.

Table 1. The powers applied to the Yb2O3 and Er2O3 targets and the corresponding measured PIXE area of Yb Lα1,2 and Er Lα1,2 (expressed in counts × keV) and estimated atomic concentration (in atomic percentage) for all the (Yb1-x-Erx)2Si2O7 thin films

All the as-grown films have a disilicate composition with an extended range of Er concentration (and Yb one accordingly), spanning from 0.2 at.% (corresponding to about 5 × 1020 at/cm3) to 18.0 at.% (corresponding to about 1022 at/cm3).

After the thermal treatment at 1200 °C for 30 s in O2 ambient the disilicate films crystallize in the α-crystalline phase (XRD spectra not shown), as already known for Er and (Y-Er) disilicates [9, 15-16]. Moreover it has been demonstrated that in the (Y-Er) disilicate Er3+ can be substituted by Y3+ ions [2, 8]. Also all the (Yb1-xYx)2Si2O7 and (Yb1-xErx)2Si2O7 thin films crystallize in α-phase even by varying the relative REs amount. This confirms that also in these two disilicates we can insert gradually Yb3+ ions in substitution of Er3+ (or of Y3+ ions), thanks to the similarities of Y, Yb and Er ionic radii and of their chemical properties.

3.2 Optical properties: Yb-Er coupling

We have investigated the photoluminescence properties of α-(Yb1-xErx)2Si2O7 thin films by exciting all the samples under low pump flux (ϕ = 1.6 × 1019 cm−2s−1) at room temperature. In order to study the excitation mechanisms of Er3+ and Yb3+ ions we have also analyzed the PL properties of both REs in some reference samples, α-(Y1-xErx)2Si2O7 and α-Yb2Si2O7. Since the Y3+ ions are optically inactive, these two samples permit to distinguish the role of each single RE without the influence of the other one. By exciting under 980 nm the typical PL peak of Er3+ in α-crystalline phase [15] is observed from α-(Y1-xErx)2Si2O7, as shown in Fig. 2(a)
. By varying the excitation wavelength, λexc, between 750 nm and 1000 nm, the photoluminescence excitation (PLE) spectrum recorded at 1535 nm is characterized by two sharp peaks depicted in Fig. 2(b). The peaks centered respectively at 800 nm and at 980 nm correspond to the 4I15/2→4I9/2 and 4I15/2→4I11/2 Er3+ transitions. This is true because Er3+ ions can be excited only by direct excitation, since Y3+ ions are optically inactive.

Fig. 2PL spectra (a) from α-(Y1-xErx)2Si2O7 under λexc = 980 nm, (c) from α-Yb2Si2O7 and (e) from α-(Yb1-xErx)2Si2O7 under λexc = 920 nm. In addition we reported the PLE spectra recorded (b) at 1535 nm from α- (Y1-xErx)2Si2O7, (d) at 1025 nm from α-Yb2Si2O7 and (f) at 1535 nm from α-(Yb1-xErx)2Si2O7. The inserts report the levels schemes of the Er3+ and Yb3+ ions. Note that spectrum (b) has been multiplied by a factor of 10. This same amplification factor has been used in (f) in the wavelength range 750-850 nm.

The PL properties of Yb3+ ions have been instead studied in α-Yb2Si2O7 (NYb = 18 at.%). Under λexc = 920 nm the PL emission has the typical broad band shape between 900 and 1100 nm associated to the 2F5/2→2F7/2 Yb3+ de-excitation, see Fig. 2(c). A further band has been found between 1100 and 1200 nm due to the band edge recombination of carriers in Si. Therefore we have measured the PLE spectrum by recording the PL intensity at 1025 nm, shown in Fig. 2(d). It is characterized by a main peak at 980 nm and a further band around 920 nm, both associated to the 2F7/2→2F5/2 transitions of Yb3+ ions typically observed in several other matrices [10–13, 17–20].

When we excite the α-(Yb1-xErx)2Si2O7 under λexc = 920 nm, that is not resonant with any Er3+ transitions, the PL spectrum, in Fig. 2 (e), shows not only the Yb3+ emission between 900 and 1200 nm but also the typical peak of Er3+ at 1535 nm. Therefore in this case Er3+ can be excited only by Yb3+ sensitization, thanks to the good matching between the Yb3+ 2F5/2 and the Er3+ 4I11/2 levels and the thermalization process within the Yb3+ system, possibly involving migration of excitation [18,19]. In order to understand the role of Yb3+ in the Er3+ excitation mechanism the PLE spectrum recorded at 1535 nm has been analyzed, as reported in Fig. 2(f). It is worth noticing that its shape is a superposition of the two PLE spectra already described for Er3+ in α-(Y1-x-Erx)2Si2O7 and for Yb3+ in α-Yb2Si2O7. We can conclude that Er3+ in α-(Yb1-xErx)2Si2O7 can be excited both by the direct absorption and by the mediated excitation from Yb3+ ions.

Moreover by comparing the PL intensity at 1535 nm of the two samples having the same Er3+ content, in presence (Fig. 2(f)) and in absence (Fig. 2(b)) of Yb3+, a very similar signal is observed under λexc = 800 nm. Instead under λexc = 980 nm PL at 1535 nm is surprisingly one order of magnitude higher when Er3+ ions are in presence of Yb3+, in the α-(Yb1-xErx)2Si2O7 (note that in Fig. 2(b) the whole spectrum is multiplied by a factor of 10). This is due to the superposition of the two contributions, the direct Er3+ excitation and the mediated excitation from Yb3+ ions.

By comparing all the α-(Yb1-xErx)2Si2O7 thin films we have observed very similar PLE shapes. Therefore the relevant mediated contribution observed for all the samples demonstrates the efficient Yb-Er energy transfer also for the lowest NYb values. By increasing NYb the PL signal observed under mediated excitation condition increases with respect to the PL recorded under λexc = 980 nm that is resonant with both the direct and the mediated excitation. The ratio of the PL signals at 1535 nm recorded under pure mediated excitation (λexc = 935 nm) and under resonant plus mediated excitation (λexc = 980 nm) can give important quantitative information on the Er3+ excitation mechanisms. In fact for pure resonant excitation this ratio should be zero, while for pure mediated excitation could be 0.45 (as for the emission of a pure Yb sample, Fig. 2(d)). The PL(λexc = 935 nm)/PL(λexc = 980 nm) ratio is reported as a function of NYb (or NEr, upper scale) in Fig. 3
.

Fig. 3PL(λexc = 935 nm)/PL(λexc = 980 nm) ratio recorded at 1535 nm as a function of NYb, bottom scale, or NEr, top scale (NEr + NYb = 18 at.%). In the right hand scale is reported the estimation of mediated contribution.

This ratio raises by increasing NYb and it reaches its maximum value for all the samples containing NYb higher than 10.0 at.% (or NEr lower than 8.0 at.%). It is very interesting to note that this maximum value corresponds exactly to the ratio PL(λexc = 935 nm)/PL(λexc = 980 nm) recorded at 1025 nm from Yb3+ ions in the α-Yb2Si2O7 thin films. This value is therefore characteristic of Yb3+ and it comes from the dependence of its absorption cross section on λexc. By normalizing the PL ratios detected at 1535 nm from the α-(Yb1-xErx)2Si2O7 to the maximum value we obtain an estimation of the percentage mediated contribution under λexc = 980 nm. It means that for the regime with NYb> 10.0 at.% (and NEr< 8.0 at.%), 100% mediated contribution is reached and the PL at 1535 nm is mainly due to the sensitization of Er3+ ions from Yb3+ ones. This regime corresponds to the condition for which the Yb3+ donors overcome the Er3+ acceptors.

In order to quantify the energy-transfer efficiency in α-(Yb1-xErx)2Si2O7 we have analyzed the Yb3+ decay rate, WYb-Er. In fact if an Yb-Er interaction is present, then it is expected that new non-radiative channels increase the Yb3+ intrinsic decay rate, WYb, under the relationship

We have estimated WYb-Er and WYb (inverse of the lifetime) by analyzing the PL decay curves recorded at 980 nm under λexc = 935 nm respectively from the α-(Yb1-xErx)2Si2O7 and α-(Yb1-x-Yx)2Si2O7 films having the same Yb contents (see Fig. 4(a)
). By single exponential fits we have estimated the decay times for all the samples and then extracted the respective rates. WYb-Er was found to be always 0.1 μs−1, much higher than the typical radiative decay rate of Yb (0.7 ms−1) [17]. On the other hand WYb was found to be strongly dependent on NYb. By increasing NYb from 1.5 at.% to 17.8 at.%, WYb increases from 3.9 ms−1 to 62 ms−1, but always lower than WYb-Er. The WYb trend is due to the energy migration among Yb3+ ions and their interaction with impurities, such as OH- radicals [17], dependent respectively on the square of NYb and on NYb.

Then η is evaluated for each NYb according to Eq. (2) and the obtained values are reported in Fig. 4(b). Note that η is almost 1 for 1.5 Yb% (and 16.5 Er%). This means that for the lowest NYb all the power absorbed by Yb3+ ions is lost by energy transfer to Er3+ ions, given that for each donor there are 8 available acceptors. By increasing NYb η slightly decreases down to 0.4, because the excited Yb3+ fraction that efficiently transfers to a nearby Er3+ ion is decreasing as well. This is due to the stronger Yb-Yb interaction, demonstrated by the WYb increase, at the expenses of the Yb-Er coupling. However η decreases only by about a factor of 2.4 by rising NYb by a factor of 10. This means that the number of excited Yb3+ ions that efficiently transfer to Er3+ are still increasing by a factor of 4, corresponding to 6.0 × 1021 sensitizers/cm3. Such a number is much larger than the available acceptors, 1.6 × 1020 Er/cm3.

The energy-transfer coefficient, CET, is obtained through Eq. (1) and it is reported in the right hand scale of Fig. 4(b). It has a slight increase by increasing NYb and a sharp increase for NYb> 10 at.%, reaching a value of about 2 × 10−16 cm−3s−1. Since we have demonstrated that the actual sensitizers are always more than the acceptors, the simultaneous decrease of NEr implies that each Er3+ feels a stronger coupling with Yb3+ population. The maximum CET value is very high and comparable to the best energy-transfer coefficient reported in literature in other phosphate glasses containing typically about 1019 Er/cm3 and 1020 Yb/cm3 [21]. Therefore we have demonstrated that Er3+ excitation through Yb3+ is still efficient in (Yb-Er) disilicates even by increasing the RE concentrations up to one order of magnitude.

3.3 Optical properties: efficient PL emission from Er3+ ions

When Er3+ ions in α-(Yb1-xErx)2Si2O7 are excited to the second excited level, 4I11/2, they fastly decay to the first excited level, 4I13/2, and then radiatively to the ground state, 4I15/2, by emitting a photon at 1535 nm [9]. In order to study the Er3+ emission properties we have measured the decay time, τ1,Er, as a function of NYb (and NEr) under λexc = 980 nm. Figure 5(a)
shows some traces at different Er contents taken at low pump flux of 1.6 × 1019 cm−2s−1. In the same figure the decay curve in absence of Yb (in Y-Er disilicates) is also reported for a particular NEr, 1.6 at.%. It is interesting to note that the Er3+ lifetime depends only on Er3+ content and is not affected by the presence of Yb (or Y).

Fig. 5(a) PL decays recorded at 1535 nm for α-(Yb1-x-Erx)2Si2O7 at different NEr. For NEr = 1.6 at.% the decay curve for α-(Y1-x-Erx)2Si2O7 is also reported. Continuous black lines are the single exponential fits of the data. (b) PL intensity (left hand scale) and lifetime (right hand scale) at 1535 nm as a function of NYb (bottom scale) in α-(Yb1-x-Erx)2Si2O7. For comparison the decay times (black open triangles) at 1535 nm in α-(Y1-x-Erx)2Si2O7 as a function of NEr (top scale) have been reported. The blue line is a guide for the eye. The measurements of both panels are obtained under λexc = 980 nm and at ϕ = 1.6 × 1019 cm−2s−1.

The lifetime values, τ1,Er, have been evaluated for all the samples by single exponential fits of the decay curves and reported in the right hand scale of Fig. 5(b) (blue line and open squares). It is evident that by decreasing NEr, τ1,Er varies between 0.3 ms and 5.6 ms. The same trend is observed in the case of α-(Y1-xErx)2Si2O7 samples (black open triangles). Therefore the τ1,Er behavior can be associated only to the Er-Er interactions not involving Yb3+ ions. For the highest NEr, the very short lifetime can be justified by the occurrence of concentration quenching between Er3+ ions owing to the short mean Er-Er distance. This phenomenon consists in a resonant energy transfer from one excited Er3+ ion at the first excited level to a nearby Er3+ ion in the ground state. Hence energy travels along the sample by eventually being lost when a quenching center is encountered [2]. By increasing NYb and, as a consequence, by decreasing NEr the deleterious Er-Er interactions are reduced. It means that the decay time of the 4I11/2→4I13/2 transition and the Yb-Er energy-transfer time are faster than the Er3+ de-excitation time from the 4I13/2 level. In this α-(Yb1-xErx)2Si2O7 host the good Yb-Er coupling and the reduced back-transfer from Er3+ to Yb3+ are guaranteed for the whole NEr range under investigation.

In Fig. 5(b) data on the PL intensity at 1535 nm as a function Yb and Er contents are also summarized (left hand scale). A very strong PL emission has been already demonstrated from Er3+ in Er2Si2O7 (NYb = 0 at.%) [3], because all Er3+ ions are optically active. By introducing just 2 at.% of Yb, PL intensity at 1535 nm already increases by a factor of three though the emitting Er3+ ions are reducing from 18 at.% to 16.5 at.%. This is due to the occurrence of mediated excitation from Yb3+ ions as already demonstrated. The PL intensity continues to increase by further increasing NYb. This trend cannot be justified only by the simultaneous slight increase of τ1,Er but it is mainly due to the raising of the mediated contribution at λexc = 980 nm (see Fig. 3). When we reach the maximum mediated excitation contribution for NYb> 10 at.% the PL intensity continues to increase, reaching its maximum for 1.6 Er% and 16.4 Yb%. This further increase is justified by the strong increase of τ1,Er that compensates the reduction of the number of Er3+ emitting centers. By further increasing NYb, though the maximum radiative efficiency and the maximum CET (see Fig. 4(b)) is reached, the number of Er3+ ions available for excitation is too low. This determines that the overall Er emission at 1535 nm drops down.

For the sample having the best PL emission at 1535 nm the PL trend has been recorded as a function of pump flux, ϕ, in the range between 1018 cm−2s−1 and 1022 cm−2s−1 as reported in Fig. 6
. A comparison with the α-(Y1-x-Erx)2Si2O7 sample with the same Er content, but in absence of Yb, is also shown. In the two cases the PL trend is very similar, it increases linearly and then it exhibits a sublinear regime. This is associated with the occurrence of up-conversion phenomena where two Er3+ ions both excited at the 4I13/2 level interact with one being de-excited to the 4I15/2 ground state and the other being resonantly excited to the 4I9/2 level [2] thus determining a depletion of the first excited level (as reported in the scheme of Fig. 6).

Fig. 6Photoluminescence intensity at 1535 nm as a function of ϕ for the α-(Yb1-xErx)2Si2O7 and the α-(Y1-xErx)2Si2O7 film having the same NEr. The excitation wavelength is λexc = 980 nm. The continuous curves are fits of the experimental data, obtained by using Eq. (3). A scheme of the up-conversion phenomenon is depicted on the right-hand corner.

In order to fit the data, the PL intensity has to be correlated with the Er3+ population in the first excited level, N1,Er. As already pointed out, Er3+ quickly de-excites from the 4I11/2 level to the 4I13/2 one and the back-energy transfer probability to the Yb3+ ions is negligible. This allows to describe the Er3+ dynamics as a two-levels system. Therefore, by solving the rate equations in steady state conditions, the population N1,Er is obtained as a function of ϕ, as reported in literature [2, 9]:

In this expression σEr is the Er3+ effective excitation cross section under λexc = 980 nm, τ1,Er is the lifetime in absence of up-conversion and Cup is the up-conversion coefficient. Note that since at ϕ = 1.6 × 1019 cm−2s−1 up-conversion is negligible, for τ1,Er we have used the value reported in Fig. 5(b). In particular, Er3+ has the very same lifetime in the two considered hosts. Moreover, since the PL intensity is proportional to N1,Er, the correlation can be done with a proper calibration procedure reported in [9] and by using a standard reference sample of Er-doped silica (see right-hand scale in Fig. 6).

Now it is possible to fit the curves of PL intensity as a function of ϕ with Eq. (3), thus obtaining the values of σEr, Cup for both samples. We have found σEr values of 2.0 × 10−21 cm2 in α-(Y1-xErx)2Si2O7, corresponding to the typical absorption cross section of Er3+ ions under λexc = 980 nm observed in silica glasses [22], and 2.0 × 10−20 cm2 in α-(Yb1-xErx)2Si2O7. This further confirms that Er3+ ions are indeed excited with an excitation cross section one order of magnitude higher if in presence of Yb3+ ions. Moreover it is very interesting to note that in this case σEr is indeed equal to the direct Yb3+ absorption cross section, as measured in Yb2Si2O7 (data not shown) and also reported in literature [17], thus confirming the efficient excitation of Er3+ ions by Yb-Er energy transfer.

As far as the Cup estimation is concerned, we have obtained the same value (6 × 10−19 cm3/s) in the two cases and this is a direct proof that the interactions between Er3+ ions in the first excited level are not influenced by the presence of Yb3+. Moreover this value is of the same order of magnitude of that reported for Er-doped silica [23] or for (Y-Er) silicate [8,9].

The higher σEr and similar Cup for Er3+ in (Yb-Er) disilicate determine that the PL intensity is always higher than the one in α-(Y-Er) disilicate for the whole ϕ range. Moreover it is very interesting to note that in α-(Yb1-x-Erx)2Si2O7 almost 20% of Er3+ ions can be excited at the first excited level at photon fluxes of about 1023 cm−2s−1. This excited fraction is almost one order of magnitude higher than the one obtained in the case of Er3+ in absence of Yb3+. This remarkable result demonstrates that the α-(Yb1-xErx)2Si2O7 system is a very promising material for optical amplifiers at 1535 nm.

4. Conclusion

We have demonstrated that by introducing simultaneously Yb3+ and Er3+ inside an α-disilicate an efficient coupling between the two RE ions is possible for an extended range of RE concentrations. Though the high RE contents introduce deleterious non-radiative paths in all the cases Yb3+ ions can efficiently transfer energy to Er3+. In particular, for NYb increasing from 0 at.% to 10 at.% the effective excitation of Er3+ passes from 2.0 × 10−21 cm2 (direct Er3+ photon absorption) to 2.0 × 10−20 cm2 (Yb3+ mediated energy transfer). With further increasing Yb concentration (and consequently decreasing Er) up to 16.4 at.% the PL intensity at 1535 nm further increases due to a decrease of Er-Er concentration quenching. These results are made possible by the high phonon energy that characterizes the host thus guaranteeing high probability for the 4I11/2→4I13/2 relaxation and therefore a reduced back-energy transfer from Er3+ to Yb3+, favouring the population of the 4I13/2 level. Though the strong Yb3+ absorption cross section allows an efficient Er3+ excitation, in waveguide applications Yb3+ itself could hinder the overall amplifying performance. In fact, given the higher absorption cross section of Yb3+, the propagation length of the pumping signal through the waveguide is lower than in the case of a waveguide with Er3+ only, thus shortening the overall length for which the 1535 nm signal can be amplified [19]. The study of the Yb-Er interaction in such (Yb-Er) silicates in which the Er content can be continuously increased is then fundamental to find a trade-off between the waveguide length and the absolute number of excited Er3+ ions. Moreover the very high excited population at 4I13/2 level obtained for the optimized (Yb1-xErx)Si2O7 film (having 1.6 Er%, corresponding to 1.4 × 1021 Er/cm3) makes this material very promising as active medium for amplification at 1535 nm.

Acknowledgements

The authors wish to thank G. Franzò and F. Iacona for discussions, L. Romano and M. G. Grimaldi for several contributions on RBS and PIXE analyses, C. Percolla and S. Tatì for expert technical assistance.

PL spectra (a) from α-(Y1-xErx)2Si2O7 under λexc = 980 nm, (c) from α-Yb2Si2O7 and (e) from α-(Yb1-xErx)2Si2O7 under λexc = 920 nm. In addition we reported the PLE spectra recorded (b) at 1535 nm from α- (Y1-xErx)2Si2O7, (d) at 1025 nm from α-Yb2Si2O7 and (f) at 1535 nm from α-(Yb1-xErx)2Si2O7. The inserts report the levels schemes of the Er3+ and Yb3+ ions. Note that spectrum (b) has been multiplied by a factor of 10. This same amplification factor has been used in (f) in the wavelength range 750-850 nm.

(a) PL decays recorded at 1535 nm for α-(Yb1-x-Erx)2Si2O7 at different NEr. For NEr = 1.6 at.% the decay curve for α-(Y1-x-Erx)2Si2O7 is also reported. Continuous black lines are the single exponential fits of the data. (b) PL intensity (left hand scale) and lifetime (right hand scale) at 1535 nm as a function of NYb (bottom scale) in α-(Yb1-x-Erx)2Si2O7. For comparison the decay times (black open triangles) at 1535 nm in α-(Y1-x-Erx)2Si2O7 as a function of NEr (top scale) have been reported. The blue line is a guide for the eye. The measurements of both panels are obtained under λexc = 980 nm and at ϕ = 1.6 × 1019 cm−2s−1.

Photoluminescence intensity at 1535 nm as a function of ϕ for the α-(Yb1-xErx)2Si2O7 and the α-(Y1-xErx)2Si2O7 film having the same NEr. The excitation wavelength is λexc = 980 nm. The continuous curves are fits of the experimental data, obtained by using Eq. (3). A scheme of the up-conversion phenomenon is depicted on the right-hand corner.

Tables (1)

Table 1 The powers applied to the Yb2O3 and Er2O3 targets and the corresponding measured PIXE area of Yb Lα1,2 and Er Lα1,2 (expressed in counts × keV) and estimated atomic concentration (in atomic percentage) for all the (Yb1-x-Erx)2Si2O7 thin films

Metrics

Table 1

The powers applied to the Yb2O3 and Er2O3 targets and the corresponding measured PIXE area of Yb Lα1,2 and Er Lα1,2 (expressed in counts × keV) and estimated atomic concentration (in atomic percentage) for all the (Yb1-x-Erx)2Si2O7 thin films